Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 3. Preparation of o-methoxyphenyl- and o-methylsulfanylphenyl-alkynes and their cyclisation to benzofurans and benzothiophenes

Fourteen new beta-oxo phosphorus ylides 1-14 bearing o-methoxybenzoyl or o-(methyl-sulfanyl)benzoyl groups have been prepared and their pyrolytic behaviour studied. While flash vacuum pyrolysis (FVP) at 700 degrees C brings about extrusion of Ph(3)PO to give the expected alkynes 16, this is accompanied at 850 degrees C by loss of Me(.) and cyclisation of the resulting radicals to afford 2-substituted benzofurans or benzothiophenes 17-24. Where the substituent R(1) on the ylidic carbon of the starting material is phenyl, this is incorporated unchanged into the heterocyclic products. Where R(1) is Et or Pr-i the vinyl products are formed by intramolecular abstraction of a B-hydrogen atom following cyclisation. For R(1) = Me, Pr, Bu and C5H11 the cyclisation is followed by hydrogen atom abstraction from the alkyl group leading to its fragmentation and giving products with 2-methyl, ethyl and vinyl substituents. In these cases the products can be accounted for by a radical chain reaction involving the unusual homolytic substitution of a carbon radical at a saturated carbon atom.